. ii.] SIGHT. 517 



it is found that the purple colour ia confined exclusively to the 

 rods and to the outer limbs of the rods, the inner limbs being wholly 

 devoid of it. 



The colour of the rods is due to the presence of a distinct 

 pigment, the "visual purple," diffused through the substance of the 

 outer limbs ; and this may be extracted from the rods by dissolving 

 these in an aqueous solution of bile salts. A clear purple solution 

 is thus obtained, which is capable of being bleached by the action 

 of light, and in its general features and behaviour is similar to the 

 pigment as it naturally exists in the retina. 



Visual purple is found as we have said exclusively in the outer 

 limbs of the rods; it has never yet been found in the cones, 

 and it is accordingly absent from "the retinas (such as those of 

 snakes) which are composed of cones only, and from the macula 

 lutea and fovea centralis of the retinas of man and the ape. The 

 intensity of the colouration varies in different animals, and the 

 retinas even of some animals possessing rods (bat, dove, hen) seem 

 to be wholly devoid of the visual purple; it is generally well 

 marked in retinas in which the outer limbs of the rods are well 

 developed. Its absence or presence is not dependent on nocturnal 

 habits, since the intense colour of the retina of the owl is in strong 

 contrast to the absence of colour in the bat. It has been found in 

 the retina of the embryo. 



The visual purple is bleached not only by white but also by 

 monochromatic light, Of the various prismatic rays the mos't 

 active are the greenish-yellow rays, those to the blue side of these 

 coming next, the least active being the red. Now it is precisely 

 the greenish-yellow rays which are most readily absorbed by the 

 colour itself. A natural coloured retina or a solution of visual 

 purple gives a diffuse spectrum without any defined absorption 

 bands, and according to the amount of colouring material through 

 which the light passes, absorption is seen either to be limited to 

 the greenish-yellow part of the spectrum or to spread thence 

 towards the blue and, to a much less extent, towards the red. 

 Thus the various prismatic rays produce a photochemical effect on 

 the visual purple in proportion as they are absorbed by it. Under 

 the action of light the visual purple, whether in solution, or in its 

 natural condition in the rods, passes through a purplish orange to 

 a yellow, and finally becomes colourless ; and we appear to be 

 justified in speaking of a "visual yellow" and "visual white" as 

 products of the photochemical changes undergone by the visual 

 purple. 



For the restoration of the visual purple, after it has been 

 destroyed by light, the maintenance of the circulation of the blood 

 through the tissues of the eye is not essential. The choroidal 

 epithelium has by itself, provided that it still retains its tissue life, 

 the power of regenerating the purple. If a portion of the retina 

 of an excised eye be raised from its epithelial bed, bleached, and 



